1. Field of the Invention
The present invention relates to a hydraulic circuit construction for a working vehicle equipped with a backhoe system, a dozer system and right and left running systems, in which a first hydraulic motor for one of the running systems and a hydraulic cylinder for the dozer system are adapted to be operated by a first hydraulic pump, a second hydraulic motor for the other running system and a hydraulic cylinder for an arm of the backhoe system are adapted to by operated by a second hydraulic pump, and further said arm cylinder is adapted to receive pressure fluid from the first and second pumps at the same time. The circuit construction comprises a multiplex valve assembly including a control valve for the second motor and a control valve for the arm cylinder connected in parallel with each other to the second pump, and a further multiplex valve assembly including a control valve for the dozer cylinder, a control valve for the first motor and a confluence control valve for supplying the pressure fluid from the first pump to the arm cylinder.
2. Description of the Prior Art
The above-described circuit construction has an advantage that the piping system and the valve arrangement are simple because of the employment of the multiplex valve assemblies resulting in an extremely compact overall piping system, and a further advantage that a higher operational efficiency of the arm of the backhoe system is achieved by incorporating into the hydraulic system for the first pump the confluence circuit for the arm cylinder to be primarily operated by the second pump.
On the other hand, in order to employ a multiplex valve assembly, all control valves therein have to be connected, selectively, either in parallel or in series with one another to the pump common to them. Accordingly, said three control valves in the further multiplex valve assembly, namely the dozer, first motor and arm confluence valves, have also to be connected either in parallel or in series with one another to the first pump.
In case the three valves are connected in parallel, the following problem occurs when bulldozing and running operations are conducted at the same time (the backhoe system stays idle then).
In this case, the revolution speed of the first motor becomes lower than that of the second motor as the pressure fluid from the second pump is wholly supplied to the second motor whereas the pressure fluid from the first pump is divided to the dozer cylinder and to the first motor, which results in an oblique running of the working vehicle.
On the contrary, in case that the three valves are connected in series, the above-described problem when bulldozing and running operations at the same time is solved, but another problem occurs when, for example, attempting to drive the vehicle out of a marshy ground by operating both motors and the arm of the backhoe system at the same time with the bucket engaged in the ground (the dozer system stays idle then). That is to say, in such a case the second motor can be held in operation as the second motor and the arm cylinder are connected in parallel with each other to the second pump whereas the first motor becomes inoperative by the work load on the arm, which results in a running state of the vehicle with only one running system operated. With the running systems thus operated only on one side, the vehicle would be unable to run out of the marshy ground as desired, while other objections would occur such as release of the pressure fluid, rise in the temperature of the fluid or loss of the engine power.
It will be clear from the above description that prior art working vehicles incorporating multiplex valve assemblies because of their advantages are unable to achieve one of both types of operations, namely a bulldozing operation without the vehicle being urged to run obliquely or a successful running out operation from a marshy ground by operating the arm of a backhoe system without any one of the running systems being urged to stop.